Carbon and nitrogen abundance variations on the main sequence of 47 Tucanae

Abstract

The FOCAP multi‐object fibre optic system on the AAT has been used to obtain spectra for a sample of more than a hundred faint stars in the globular cluster 47 Tuc (NGC 104), from the base of the giant branch to about a magnitude below the main‐sequence turn‐off. The spectra cover the ultraviolet and blue bands of CN and the G band of CH. Quantitative abundances have been derived by comparing the observations with synthetic spectra. The main‐sequence stars can be divided into two approximately equal groups, one having N enhanced by a factor of ∼ 7, and C depleted by 40 per cent, relative to the other, similar to what is inferred for the red giants. The mean heavy‐element abundance, as measured by Ca or Fe, differs between the two groups by no more than 0.01 ± 0.05 dex. All the available abundance data indicate that the atmospheres of 47 Tuc stars contain different amounts of material which has been processed through the CNO cycle, but not through more advanced stages of nucleosynthesis. In the red giants, this could be a result of the convective dredge‐up of processed material from deep within the stars themselves, as occurs in some more metal‐poor clusters. However, stars on the upper main sequence of globular clusters do not have convective envelopes, while any deep mixing there would have consequences incompatible with other observational data. Thus it seems that the main‐sequence C and N variations must be either primordial or caused by a pollution mechanism. The similarity of the bimodal abundance patterns among the dwarfs and giants then suggests that the dredge up of CNO‐cycled material is not the dominant process in the red giants either, at least not in 47 Tuc. The CNO‐processed material seems most likely to have come from intermediate‐mass stars on the asymptotic giant branch; various mechanisms by which such material might appear in the main‐sequence stars are discussed. Explanations involving a truly primordial origin, or having successive generations of stars, seem to require rather special ‘tuning’. A somewhat speculative alternative self‐enrichment process is proposed, in which stellar winds from intermediate‐mass stars were captured by existing low‐mass stars, early in the life of the cluster. Whatever the mechanism, the abundance data presumably contain important clues about the formation and early evolution of globular clusters.

Red Giant Branch Stars: The Theoretical Framework

“…While for the first cluster the comparison between TO and SGB stars shows no significant abundance differences and a very low scatter in oxygen, NGC 6752 already displays the full range of the O-Na anticorrelation close to the main sequence. These observations provide strong evidence for a nonevolutionary source for the abundance anomalies, as do the results by King et al (1998) Cannon et al 1998;M4, Ivans et al 1999;M71, Briley & Cohen 2001) a bimodal C-N abundance distribution is present all along the cluster sequence, or-as in M5 (Ivans et al 2001)-the extent of the anomaly may even be anticorrelated with evolutionary state. All these facts are interpreted in terms of the presence of a primordial source such as contamination by more massive stars.…”

Commission 37: Star Clusters and Associations: (Amas Stellaires et Associations)

“…To first order the band strength variations are bimodal and anti-correlated with the numbers of CN-strong/CH-weak and CN-weak/CH-strong stars being approximately equal. These results are similar to those found by Cannon et al (1998) for main sequence stars in 47 Tuc and they remove any possibility that 47 Tuc could be considered unique or a special case.…”

Age as a major factor in the onset of multiple populations in stellar clusters

“…However, we can still provide a rough estimate, by assuming that MPs in younger clusters are the same as those of ancient GCs, i.e. an enhancement in N is associated with a depletion in C (Cannon et al 1998 cannot exclude the presence of a slight N variation due to observational uncertainties and decontamination from field stars (see §4). If we assume a [N/Fe] ∼ +0.5 dex for NGC 1978, then we can set an upper limit to any enrichment in the younger clusters (NGC 419, 1783(NGC 419, , 1806(NGC 419, , 1846 of [N/Fe]< +0.3 dex.…”

“…However, observational evidence collected in the last decade has conclusively demonstrated that GCs host multiple populations (MPs) of stars, which manifest in the form of distinctive star-to-star light element variations. While some GC stars have the same C, N, O, Na abundances as field stars with the same metallicity (i.e., iron content), a significant fraction of cluster members show systematically enhanced N and Na along with depleted C and O (Cannon et al 1998;Carretta et al 2009). Almost all old (> 10 Gyr) and massive (above a few times 10 4 M ) GCs in the Milky Way (Piotto et al 2015) and nearby galaxies (Mucciarelli et al 2009;Dalessandro et al 2016;Niederhofer et al 2017b;Larsen, Brodie & Strader 2012) have been found to host star-to-star chemical anomalies.…”